Fundus image acquiring apparatus and control method therefor

TECHNICAL FIELD

The present invention relates to a fundus image acquiring apparatus and a control method therefor. In particular, the present invention relates to a fundus image acquiring apparatus for measuring eye movement, and a control method therefor.

BACKGROUND ART

In recent years, an apparatus for measuring eye movement has received attention. If the eye movement can be measured, such technology is applied to a vision field test or a fundus tomographic image acquiring apparatus for obtaining a higher resolution image, and hence a more accurate fundus test can be performed.

In order to measure the eye movement, there are various methods such as a corneal reflex method (Purkinje image) and a search coil method. Among others, a method of measuring eye movement from a fundus image is being studied, which is simple and imposes little load on a subject.

In order to measure eye movement accurately using a fundus image, it is necessary to extract a characteristic point from the fundus image, to search for and detect a characteristic point in the image to be processed, and then to calculate a movement amount of the characteristic point. These steps need to be processed at high speed. The step of extracting the characteristic point is important from a viewpoint of measurement stability, accuracy, and reproducibility of eye movement. As the characteristic point of the fundus image, a macula lutea, an optic papilla (hereinafter referred to as papilla), or the like is used. Because an affected eye or the like has an incomplete macula lutea or papilla in many cases, a blood vessel may be used as the characteristic point of the fundus image. A method of extracting a characteristic point of a blood vessel is disclosed in Patent Literature 1.

CITATION LIST

Patent Literature

PTL 1: Japanese Patent Application Laid-Open No. 2001-70247

SUMMARY OF INVENTION

Technical Problem

In order to detect eye movement accurately from a fundus image, the method as described in Patent Literature 1 is used for extracting a characteristic point of the fundus, and images are compared so that an eye movement amount between the obtained images can be calculated.

However, if luminance of the characteristic point such as a blood vessel cannot be secured sufficiently, there is a problem that it takes time to extract the characteristic point or that the extraction is failed, and hence tracking cannot be performed.

Therefore, an object of the present invention is to obtain fundus images having sufficient luminance for tracking.

In addition, another object of the present invention is to set an image acquiring condition corresponding to luminance of the extracted characteristic point.

Further, another object of the present invention is to perform a tracking process by using a characteristic point having appropriate luminance.

Solution to Problem

In order to solve the above-mentioned problem, a fundus image acquiring apparatus of the present invention is a fundus image acquiring apparatus having a function of detecting movement of an eye to be inspected by using a characteristic image extracted from a fundus image of the eye to be inspected. The fundus image acquiring apparatus includes: fundus image acquiring unit for taking a first fundus image based on a first image acquiring condition; extraction unit for extracting a characteristic image from the first fundus image; evaluation unit for evaluating the extracted characteristic image; and setting unit for setting a second image acquiring condition based on a result of the evaluation by the evaluation unit, in which the fundus image acquiring unit takes a second fundus image based on the second image acquiring condition set by the setting unit.

Further, a control method for a fundus image acquiring apparatus according to the present invention is a control method for a fundus image acquiring apparatus having a function of detecting movement of an eye to be inspected by using a characteristic image extracted from a fundus image of the eye to be inspected. The control method includes: taking, by fundus image acquiring unit, a fundus image based on an initial image acquiring condition; extracting a characteristic image from the taken fundus image; evaluating the extracted characteristic image; and setting a new image acquiring condition based on a result of the evaluating, in which the taking includes taking, by the fundus image acquiring unit, a new fundus image.

Advantageous Effects of Invention

According to the present invention, the fundus images having sufficient luminance for tracking can be obtained.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a schematic diagram illustrating a structure of an optical system of a fundus camera according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 2</figref> is a schematic diagram of functional blocks of an apparatus according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 3</figref> is a control flow chart according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 4</figref> is a flow chart of a process A in the control flow according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 5</figref> is a flow chart of a process B in the control flow according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 6</figref> is a flow chart of a process C in the control flow according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 7</figref> is a flow chart of a process D in the control flow according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 8</figref> is a graph showing a frame rate and a luminance difference according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 9A</figref> is a schematic diagram illustrating a fundus image according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 9B</figref> is a schematic diagram illustrating a fundus image according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 9C</figref> is a schematic diagram illustrating a fundus image according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 9D</figref> is a schematic diagram illustrating a fundus image according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 9E</figref> is a schematic diagram illustrating a fundus image according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 10</figref> is a schematic diagram of luminance difference calculation of the fundus image according to Example 1 of the present invention.

<figref idref="DRAWINGS">FIG. 11</figref> is a schematic diagram illustrating a structure of an optical system of an OCT apparatus and an SLO apparatus according to Example 2 of the present invention.

<figref idref="DRAWINGS">FIG. 12</figref> is a schematic diagram of functional blocks of the apparatus according to Example 2 of the present invention.

<figref idref="DRAWINGS">FIG. 13</figref> is a control flow chart according to Example 2 of the present invention.

<figref idref="DRAWINGS">FIG. 14</figref> is a schematic diagram of luminance difference calculation of a fundus image according to Example 2 of the present invention.

<figref idref="DRAWINGS">FIG. 15</figref> is a flow chart of a process E in the control flow according to Example 2 of the present invention.

<figref idref="DRAWINGS">FIG. 16A</figref> is a schematic diagram illustrating an SLO fundus image according to Example 2 of the present invention.

<figref idref="DRAWINGS">FIG. 16B</figref> is a schematic diagram illustrating an SLO fundus image according to Example 2 of the present invention.

<figref idref="DRAWINGS">FIG. 16C</figref> is a schematic diagram illustrating an SLO fundus image according to Example 2 of the present invention.

<figref idref="DRAWINGS">FIG. 16D</figref> is a schematic diagram illustrating an SLO fundus image according to Example 2 of the present invention.

<figref idref="DRAWINGS">FIG. 16E</figref> is a schematic diagram illustrating an SLO fundus image according to Example 2 of the present invention.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.

EXAMPLE 1

Hereinafter, Example 1 of the present invention is described.

In this example, in order to solve the problem described above, the following example is described. That is, a fundus image was obtained, and a characteristic point (also referred to as a characteristic image) was extracted. After that, luminance of the characteristic point was calculated. Thus, eye movement was able to be measured also in the case of an eye to be inspected that was not in a good condition (for example, an affected eye), by determining a fundus image obtaining condition, namely an exposure time.

(Overall Structure of Apparatus)

A fundus image acquiring apparatus according to this example is described with reference to <figref idref="DRAWINGS">FIG. 1</figref>. A fundus camera <b>1</b> is provided with a digital single-lens reflex camera <b>50</b> that can take images at a video rate, as a fundus image acquiring unit serving as a signal obtaining unit, via a connection unit <b>40</b>. Using the digital single-lens reflex camera <b>50</b>, the exposure time is controlled so that a fundus image obtaining rate can be controlled. A perforated mirror <b>12</b> is disposed on an optical path of an objective lens <b>11</b> that is opposed to an eye E to be inspected. On the optical path in the incident direction of the perforated mirror <b>12</b>, there are arranged a relay lens <b>13</b>, a black spot plate <b>14</b>, a relay lens <b>15</b>, a ring slit plate <b>16</b>, a fluorescent exciter filter <b>17</b>, and a mirror <b>18</b>. Further, in the incident direction of the mirror <b>18</b>, there are arranged a condenser lens <b>19</b>, an imaging light source <b>20</b> constituted of a xenon tube, a condenser lens <b>21</b>, and an observation light source <b>22</b> constituted of an infrared emitting diode. Note that the optical path is indicated by a solid line in the figure.

In back of the perforated mirror <b>12</b>, there are arranged a focus lens <b>24</b>, a fluorescent barrier filter <b>25</b>, an imaging lens <b>26</b>, and the digital single-lens reflex camera <b>50</b>. Note that the entire fundus camera <b>1</b> is controlled by a control unit <b>205</b> (see <figref idref="DRAWINGS">FIG. 2</figref>, not shown in <figref idref="DRAWINGS">FIG. 1</figref>).

In the digital single-lens reflex camera <b>50</b>, there are arranged a quick return mirror <b>51</b>, a focal plane shutter (not shown), and a two-dimensional sensor <b>53</b> on the same optical path as that in the back of the objective lens <b>11</b>. In addition, in the reflecting direction of the quick return mirror <b>51</b>, a pentaprism <b>54</b> and an ocular lens <b>55</b> are disposed. A signal of light received by the two-dimensional sensor <b>53</b> is processed by a signal processing board <b>52</b>. Then, the signal is transferred to a computer (PC) <b>56</b> via a cable and is displayed on a display <b>57</b>. In addition, the PC <b>56</b> is connected to an external storage device (HDD) <b>58</b>, which stores data described later.

(Functional Block)

<figref idref="DRAWINGS">FIG. 2</figref> illustrates functional blocks of the fundus image acquiring apparatus according to this example. The fundus image acquiring apparatus includes a CPU <b>203</b> which controls the entire apparatus and also performs image processing and the like (corresponding to the PC <b>56</b> illustrated in <figref idref="DRAWINGS">FIG. 1</figref>), the control unit <b>205</b> which controls obtaining of a fundus image (inside the fundus camera <b>1</b> illustrated in <figref idref="DRAWINGS">FIG. 1</figref>), a fundus imaging unit <b>201</b> which takes a fundus image (fundus image acquiring unit, corresponding to the fundus camera <b>1</b> and the digital single-lens reflex camera <b>50</b> illustrated in <figref idref="DRAWINGS">FIG. 1</figref>), a display unit <b>202</b> which displays states of the apparatus (corresponding to the display <b>57</b> illustrated in <figref idref="DRAWINGS">FIG. 1</figref>), and an HDD (recording unit) <b>204</b> which records the fundus images, the image acquiring conditions, and the like (corresponding to the external storage device <b>58</b> illustrated in <figref idref="DRAWINGS">FIG. 1</figref>). When a fundus is observed and when an image thereof is taken, the CPU <b>203</b> issues an instruction of the image acquiring condition to the control unit <b>205</b> so that an image of the fundus is taken. After an image of the fundus is taken, the image is sent from the fundus imaging unit <b>201</b> to the CPU <b>203</b>, and image processing and the like are performed. After that, the image acquiring condition is determined, and an instruction of the image acquiring condition is issued to the control unit <b>205</b> so that an image of the fundus is taken. Then, the CPU <b>203</b> analyzes the taken images so as to measure eye movement or the like, which is displayed on the display unit <b>202</b>, and is stored in the recording unit <b>204</b> simultaneously or later.

(Process Flow)

<figref idref="DRAWINGS">FIG. 3</figref> illustrates a general flow of measuring eye movement for a constant period of time using the above-mentioned function.

First, the process starts by initializing the apparatus by an initial image acquiring condition (Step <b>301</b>).

Next, in an imaging step, using the fundus imaging unit <b>201</b>, by the initial (or first) image acquiring condition, a (first) fundus image is taken and obtained (Step <b>302</b>). The initial image acquiring condition in this example includes a frame rate of 20 Hz and a fundus diameter of 10 mm. The CPU <b>203</b> extracts a characteristic point (hereinafter referred to as a template) from the obtained first fundus image (Step <b>303</b>). The extraction step of extracting a characteristic point from the taken fundus image as described above is performed by a part of the CPU <b>203</b> having a function as extraction unit. Template information (template ID number, template image, coordinates (template coordinates) with respect to a reference position in the fundus image, and the like) of the extracted template is stored in the HDD <b>204</b>. In this example, multiple template information pieces, preferably at least three template information pieces are stored (Step <b>311</b>). After that, luminance is measured in a process C for evaluating the template image (Step <b>304</b>). Luminance of the received signal is processed by 256-gradation. Based on a result of the measurement of luminance, an appropriate frame rate is calculated, and the image acquiring condition is set based on the calculated frame rate (process D) (Step <b>310</b>). An image is obtained at the set frame rate, and in the obtained image (Step <b>305</b>), template matching (process A) is performed (Step <b>306</b>) to detect matching coordinates so that the eye movement is measured (process B) (Step <b>307</b>). The eye movement amount, the image, measurement time, a real time monitor image of an anterior ocular portion, and the like are displayed on the display unit <b>202</b> (Step <b>308</b>). The process from Step <b>305</b> to Step <b>308</b> is repeated until the measurement of eye movement is finished (until a new image is obtained). When an end of the measurement is instructed, the process is finished (Step <b>309</b>).

Next, the process A (Step <b>306</b>) as a partial flow is described with reference to <figref idref="DRAWINGS">FIG. 4</figref>. As to the template matching, the template image stored in the HDD <b>204</b> is read (Step <b>402</b>) so that the template matching is performed in a newly-obtained fundus image (Step <b>404</b>). Because the template matching is a general process, a detailed description thereof is omitted. After the template matching is finished, coordinates obtained as a result of the matching, that is, matching coordinates are stored in the HDD <b>204</b> (Step <b>405</b>). This process is performed for each template stored in the HDD <b>204</b>.

Next, the process B (Step <b>307</b>) is described with reference to <figref idref="DRAWINGS">FIG. 5</figref>. The template coordinates and the matching coordinates are read from the HDD <b>204</b> (Step <b>502</b>), and a coordinate difference is calculated for each template (Step <b>503</b>) so that an eye movement distance in the fundus image is calculated from the coordinate difference.

The process C (Step <b>304</b>) is described with reference to <figref idref="DRAWINGS">FIG. 6</figref>. The obtained template image is read (Step <b>602</b>). Luminance or luminance information of each pixel on the outermost periphery of the image is extracted (Step <b>603</b>). A luminance difference between neighboring pixels (in the present invention, the luminance difference unit a difference in value when the signal is processed by 256-gradation) is measured (Step <b>604</b>). Positions of top three spots having largest luminance differences are selected (Step <b>605</b>), and a luminance difference average of the top three spots is calculated (Step <b>606</b>). Luminance difference information (selected positions, luminance values of the positions, and calculated luminance difference average value) are stored in the HDD <b>204</b> (Step <b>608</b>). By the above-mentioned steps, the extracted template (characteristic point) is evaluated. Therefore, the CPU <b>203</b> includes a part that functions as an evaluation unit for performing the process C based on the luminance information. A result of the evaluation by the evaluation unit is once stored in the HDD <b>204</b> as storage unit. Note that the above-mentioned process may be performed in each of the multiple extracted template images or only in a particular image, for example, in a template image that is first extracted.

The process D (Step <b>310</b>) is described with reference to <figref idref="DRAWINGS">FIG. 7</figref>. The luminance difference information stored in the HDD <b>204</b> is read (Step <b>702</b>), and a frame rate is determined based on the luminance difference information (Step <b>703</b>). In other words, based on the above-mentioned result of evaluation by the evaluation unit, a new or second image acquiring condition is set. Therefore, the CPU <b>203</b> includes a part that functions as setting unit for performing the process D. As to the determination of the frame rate, for example, a table corresponding to a graph illustrated in <figref idref="DRAWINGS">FIG. 8</figref> is stored in advance, and a value of the luminance difference information is associated with the table so that an appropriate frame rate is determined. The fundus imaging unit <b>201</b> as the fundus image acquiring unit takes a new (second) fundus image based on the new image acquiring condition set by the setting unit described above, and then the tracking measurement described later is performed.

Note that the above-mentioned Step <b>303</b> corresponds to the extraction step of extracting a characteristic point from the taken initial fundus image according to the present invention. In addition, the process C corresponds to an evaluation step of evaluating the extracted characteristic point, and the process D corresponds to a setting step of setting a new image acquiring condition based on a result of the evaluation by the evaluation unit. Here, the above-mentioned initial image acquiring condition for taking the first fundus image is defined as a first image acquiring condition, and the condition for taking the new or second fundus image, which is defined in the process D, is defined as a second image acquiring condition in the present invention. However, “first” and “second” in this description mean values indicating the number of times for convenience sake, and should not be interpreted to limit further setting of the number of times or the image acquiring condition. In other words, according to the number of times of the process D or repetition thereof, a newly set condition and obtained image are defined as the second condition and image, and the condition defined as the precondition and the image obtained as the precondition are defined as the first image acquiring condition and the first image, respectively.

(Tracking Measurement: Specific Example)

A specific example corresponding to the above-mentioned process is described below.

There is described a specific example in which the fundus camera <b>1</b> was used so that tracking measurement of an affected eye was performed for 20 seconds by an optical system capable of obtaining a fundus image having a fundus diameter of 10 mm.

The fundus image obtained by the initial image acquiring condition is illustrated in <figref idref="DRAWINGS">FIG. 9A</figref>. A fundus image <b>901</b> was obtained by exposure to infrared light of 850 nm for 0.05 seconds (corresponding to a frame rate of 20 Hz). Because an affected eye was measured, a luminance difference of the fundus image was small. A template is extracted from the fundus image <b>901</b>. As the template, a crossing of blood vessels is extracted. All the crossings of blood vessels are extracted from the fundus image <b>901</b>. Because the affected eye had a small luminance difference, only one spot of the template was extracted as illustrated in <figref idref="DRAWINGS">FIG. 9B</figref> by reference numeral <b>902</b>. In the extracted template, the image luminance is evaluated. The evaluation of luminance is performed as follows. A template image <b>1001</b> is divided into multiple divided areas <b>1002</b> as illustrated in <figref idref="DRAWINGS">FIG. 10</figref>, and luminance of the outermost peripheral divided area <b>1002</b> is extracted. Then, a luminance difference between neighboring areas is determined. Luminance differences of three spots a, b, and c illustrated in <figref idref="DRAWINGS">FIG. 10</figref> were 5, 3, and 7, respectively. An average value of the luminance differences of the three spots was 5. This luminance difference is associated with the graph illustrated in <figref idref="DRAWINGS">FIG. 8</figref>, and hence the appropriate frame rate of 3 Hz is determined.

The fundus image obtained when the frame rate is set to 3 Hz is illustrated in <figref idref="DRAWINGS">FIG. 9C</figref>. In a newly-obtained fundus image <b>904</b>, luminance was secured that enabled the template matching, because the frame rate was decreased. The template matching is performed in the fundus image <b>904</b> so that a matching image <b>905</b> is detected. Template coordinates Z<b>0</b> are determined from <figref idref="DRAWINGS">FIG. 9B</figref>, matching coordinates Z<b>1</b> are determined from <figref idref="DRAWINGS">FIG. 9D</figref>, and a variation is determined from the respective sets of coordinates so that an eye movement amount is calculated. The CPU <b>203</b> includes a part that functions as a detection unit in the present invention, which detects a position or coordinates of a new template corresponding to the template as a characteristic point that is extracted prior to the newly-taken fundus image. The above-mentioned template matching illustrated in <figref idref="DRAWINGS">FIGS. 9C to 9E</figref> is repeated, so as to measure how long the eyeball has moved from the reference position during the measurement, and the above-mentioned result is displayed on a monitor in real time or after the measurement.

Here, the CPU <b>203</b> further functions as a calculation unit, so as to calculate a movement distance from the previous fundus image to the new fundus image based on the position or coordinates detected previously.

As described above, when the template matching is performed, the obtaining rate of the fundus image is optimized based on a luminance value of the template. Thus, the reliable template matching measurable even for an eye to be inspected that is not in a good condition, for example, an affected eye, can be performed.

Note that if the evaluation unit evaluates that luminance of the characteristic point is low or the luminance difference is lower than a predetermined value and hence an appropriate image cannot be obtained, the frame rate is set lower than the frame rate of the initial image acquiring condition. In addition, if it is evaluated that the luminance is high or the luminance difference is equal to or higher than the above-mentioned predetermined value, the frame rate is set higher than the frame rate of the initial image acquiring condition. In addition, the predetermined value described here unit a value related to a luminance value or a luminance difference that enables the template matching by specifying each characteristic point in the obtained template, and it is preferred that the predetermined value be set as a data table in the CPU <b>203</b> in advance together with the luminance difference to frame rate as illustrated in <figref idref="DRAWINGS">FIG. 8</figref>.

EXAMPLE 2

Hereinafter, Example 2 of the present invention is described.

In Example 2, the following example is described. That is, a scanning laser ophthalmoscope (SLO) was used for taking a fundus image, and eye movement was measured from the SLO fundus image in the same method as in Example 1. A result of the measurement of eye movement was fed back in real time to a fundus tomographic image acquiring apparatus (optical coherent tomography (OCT) apparatus) so that a high resolution 3D OCT image was obtained. Note that the optical path is indicated by a dot line in the figure below illustrating the apparatus structure.

(Structure of OCT Apparatus)

An outline of the OCT apparatus is described with reference to <figref idref="DRAWINGS">FIG. 11</figref>.

As a low coherent light source <b>1101</b>, a super luminescent diode (SLD) light source or an amplified spontaneous emission (ASE) light source can be used suitably. As a wavelength that is suitable for the low coherent light, wavelengths at 850 nm and its vicinity and at 1,050 nm and its vicinity can be used suitably for taking a fundus image. In this example, wavelengths at 850 nm and its vicinity are used. The low coherent light emitted from the low coherent light source <b>1101</b> enters a fiber coupler <b>1102</b> via a fiber and is split into a measuring beam and a reference beam. Although a structure of an interferometer using a fiber is described here, a structure using a beam splitter in a spatial light optical system may be adopted.

The measuring beam is output as a collimated beam from a fiber collimator <b>1104</b> via a fiber <b>1103</b>. Further, the measuring beam passes through an OCT scanner (Y) <b>1105</b>, relay lenses <b>1106</b> and <b>1107</b>, an OCT scanner (X) <b>1108</b>, a dichroic beam splitter <b>1109</b>, a scan lens <b>1110</b>, a dichroic mirror <b>1111</b>, and an ocular lens <b>1112</b>, so as to enter an eye e to be inspected. As the OCT scanners (X) and (Y) serving as scanning unit, a galvano scanner is used. The incident measuring beam is reflected by a retina and traces the same optical path to return to the fiber coupler <b>1102</b>.

The reference beam is guided to a fiber collimator <b>1113</b> from the fiber coupler <b>1102</b> and output as a collimated beam. The output reference beam passes through a dispersion correction glass <b>1114</b> and is reflected by a reference mirror <b>1116</b> on an optical path length variable stage <b>1115</b>. The reference beam reflected by the reference mirror <b>1116</b> traces the same optical path to return to the fiber coupler <b>1102</b>.

The measuring beam and the reference beam that have returned are combined in the fiber coupler <b>1102</b> and guided to a fiber collimator <b>1117</b>. Here, the combined light is referred to as interference light. The fiber collimator <b>1117</b>, a grating <b>1118</b>, a lens <b>1119</b>, and a line sensor <b>1120</b> constitute a spectroscope. The spectroscope converts the interference light into intensity information of each wavelength that is measured. The intensity information of each wavelength of the interference light, which is detected by each element of the line sensor <b>1120</b>, and is transmitted to a computer (not shown), in which a process is performed on the intensity information so that a tomographic image is generated.

The above-mentioned OCT apparatus corresponds to a second fundus image acquiring unit of the present invention, which functions as a tomographic image obtaining unit.

(Structure of SLO Apparatus)

Next, an optical structure of an SLO imaging unit for obtaining a fundus image is described with reference to <figref idref="DRAWINGS">FIG. 11</figref> similarly.

As a laser light source <b>1130</b>, a semiconductor laser or an SLD light source can be used suitably. A wavelength to be used in the light source is not limited as long as it can be separated from a wavelength of the low coherent light source for OCT by the dichroic beam splitter <b>1109</b>, but a near infrared wavelength range of 700 to 1,000 nm is suitably used for image quality for observing a fundus. In this example, near infrared light of 760 nm is used as the measuring beam. The laser emitted from the laser light source <b>1130</b> is output as a collimated beam from a fiber collimator <b>1132</b> via a fiber <b>1131</b> and enters a cylindrical lens <b>1133</b>. Although the cylindrical lens is used in this example, there is no particular limitation as long as the optical element can generate a line beam. A Powell lens or a line beam shaper using a diffractive optical element may be used.

The beam that is expanded by the cylindrical lens <b>1133</b> passes through the center of a ring mirror <b>1136</b> through relay lenses <b>1134</b> and <b>1135</b>, and passes through relay lenses <b>1137</b> and <b>1138</b>, to thereby be guided to an SLO scanner (Y) <b>1139</b>. As the SLO scanner (Y) <b>1139</b>, a galvano scanner is used. Further, the beam is reflected by the dichroic beam splitter <b>1109</b>, and passes through the scan lens <b>1110</b> and the ocular lens <b>1112</b>, so as to enter the eye e to be inspected. The dichroic beam splitter <b>1109</b> is adapted to transmit an OCT beam and to reflect an SLO beam. The beam entering the eye e to be inspected irradiates the fundus of the eye e to be inspected as a line-shaped beam. This line-shaped beam is reflected or scattered by the fundus of the eye e to be inspected and traces the same optical path so as to return to the ring mirror <b>1136</b>.

The ring mirror <b>1136</b> is disposed at a position that is conjugate with a position of a pupil of the eye e to be inspected. Of the back-scattered light of the line beam irradiating the fundus, light passing through a periphery of the pupil is reflected by the ring mirror <b>1136</b> and forms an image on a line sensor <b>1151</b> by a lens <b>1150</b>. The intensity information detected by each element of the line sensor <b>1151</b> is transmitted to the computer (not shown), in which a process is performed on the intensity information so that a fundus image is generated.

The SLO apparatus described above functions as a first fundus image acquiring unit for taking a fundus image for determining a frame rate in the present invention.

In this example, the SLO apparatus is described as a structure of a line scan SLO using a line beam (hereinafter referred to as an L-SLO). As a matter of course, the SLO apparatus may be a flying spot SLO. The obtaining rate of the SLO image can be changed by controlling the SLO scanner (Y) <b>1139</b>.

(Functional Blocks)

<figref idref="DRAWINGS">FIG. 12</figref> illustrates functional blocks of the system according to this example. The system includes a CPU <b>1201</b> that controls the entire system (corresponding to the above-mentioned computer), control units <b>1202</b> and <b>1203</b> that control the L-SLO and the OCT apparatus, respectively (not shown in <figref idref="DRAWINGS">FIG. 11</figref>), line sensors <b>1204</b> and <b>1205</b> that obtain the SLO image and the OCT image, respectively (corresponding to <b>1151</b> and <b>1120</b> illustrated in <figref idref="DRAWINGS">FIG. 11</figref>), a display unit <b>1206</b> that displays states of the system (not shown in <figref idref="DRAWINGS">FIG. 11</figref>), and a recording unit <b>1207</b> that records the fundus image, the image acquiring condition, and the like (hereinafter referred to as HDD, not shown in <figref idref="DRAWINGS">FIG. 11</figref>). When a fundus image is taken, the CPU <b>1201</b> issues instructions of the image acquiring condition to the control units <b>1202</b> and <b>1203</b> so that a fundus image is taken. After a fundus image is taken, the line sensors <b>1204</b> and <b>1205</b> send image signals to the CPU <b>1201</b>, and image processing is performed. After that, the fundus image is displayed on the display unit <b>1206</b>, and is stored in the HDD <b>1207</b> simultaneously or later.

(Process Flow)

<figref idref="DRAWINGS">FIG. 13</figref> illustrates a general flow of measuring eye movement during the period while the OCT apparatus is taking a tomographic image of the eyeball using the above-mentioned function. Note that in the following description, the same process as in Example 1 is described briefly.

The SLO apparatus is activated so that a fundus image (SLO image) is obtained (Step <b>1302</b>). A template is extracted from the SLO image (Step <b>1303</b>). After the template is extracted, an image and coordinates of the template information are stored (Step <b>1304</b>). Similarly to Example 1, luminance of the template image is measured (Step <b>1305</b>). An appropriate frame rate is calculated from the luminance (Step <b>1306</b>). Note that a frame rate for the SLO image and a frame rate for the OCT image have data compatibility recorded in advance, and hence this compatibility is reflected on setting of the frame rate. A scan reference position of the OCT apparatus is recorded (Step <b>1307</b>), and the measurement by the OCT apparatus is started (Step <b>1308</b>). An OCT image is obtained at the frame rate determined in the process D (Step <b>1309</b>). After that, the process A (template matching) (Step <b>1310</b>) and the process B (calculation of eye movement amount) (Step <b>1311</b>) are performed, a process E (feedback to the OCT apparatus) is performed (Step <b>1312</b>), and Steps <b>1309</b> to <b>1313</b> are repeated for the period while the OCT apparatus is measuring the tomographic image continuously. After the OCT imaging is finished, the measurement of eye movement is finished (Step <b>1314</b>).

In this example, the CPU <b>1201</b> performs the process B as the calculation unit so as to calculate the eye movement amount. The OCT control unit <b>1203</b> corrects scan positions and conditions such as scan positions of the OCT scanner (Y) <b>1105</b> and the OCT scanner (X) <b>1108</b> as the scanning unit based on the calculated eye movement amount. The CPU <b>1203</b> includes a part that functions as correction unit for performing such correction.

The process E (Step <b>1312</b>) (feedback to the OCT apparatus) is described with reference to <figref idref="DRAWINGS">FIG. 15</figref>. The CPU <b>1201</b> reads the scan position data of the OCT apparatus (Step <b>1502</b>), and calculates voltages to be applied to the OCT scanners <b>1105</b> and <b>1108</b> from the eye movement amount (Step <b>1503</b>). After the CPU <b>1201</b> transfers the input power to the OCT control unit <b>1203</b> (Step <b>1504</b>), a signal indicating movement of the scanners is confirmed (Step <b>1505</b>). Then, change information of the scan positions (information for correcting the scan positions based on the eye movement amount) is stored (Step <b>1506</b>). The changed state, the OCT image, the SLO image (display of the matching region and the template position), remaining time, and the like are displayed (Step <b>1507</b>).

(Tracking Measurement: Specific Example)

A specific example corresponding to the above-mentioned process is described below.

An image of fundus of 10 mm×8 mm is taken by the L-SLO, and the tracking measurement is performed for 20 seconds. The OCT apparatus controls the camera to work at 70 k A-scan, the B-scan image (fundus scan range of 10 mm and a laser spot diameter of 20 μm) is constituted of 1,000 lines, and 280 B-scan images and 3D images of the retina are obtained. The measurement time is 4 seconds.

<figref idref="DRAWINGS">FIG. 16A</figref> illustrates an SLO image <b>1601</b> obtained by the L-SLO. In the SLO image <b>1601</b>, as illustrated in <figref idref="DRAWINGS">FIG. 16A</figref>, blood vessels extend from the optic papilla to the edge part in a complex manner. After the SLO image <b>1601</b> is obtained, the template image is extracted from the SLO image <b>1601</b> (<b>1603</b> illustrated in <figref idref="DRAWINGS">FIG. 16B</figref>). The template image <b>1603</b> and template coordinates X<b>0</b> (−25, −200) are stored. The origin (0, 0) of the coordinates is the center of the SLO image <b>1601</b>. The luminance difference of the template image <b>1603</b> is measured. A value of the average luminance difference in this example is 100. As understood from <figref idref="DRAWINGS">FIG. 8</figref>, an appropriate frame rate of the luminance value of 100 is 70 Hz. <figref idref="DRAWINGS">FIG. 16C</figref> illustrates an SLO image <b>1604</b> obtained at the frame rate of 70 Hz. Using the SLO image <b>1604</b>, the template matching is performed. The result is illustrated in <figref idref="DRAWINGS">FIG. 16D</figref>. After a matching image <b>1605</b> is detected, coordinates X<b>1</b> (−25, −250) of the matching image <b>1605</b> are stored. After that, as illustrated in <figref idref="DRAWINGS">FIG. 16E</figref>, an eye movement amount (movement distance) is calculated from a coordinate difference between the template image <b>1603</b> and the matching image <b>1605</b>.

Measurement of luminance of the template image is described with reference to <figref idref="DRAWINGS">FIG. 14</figref>. A template image <b>1702</b> is extracted from an obtained SLO image <b>1706</b>. The extracted template image <b>1702</b> is divided by a pitch of 10 μm indicated by <b>1701</b>. Luminance of each divided area is measured. A sum of luminance values of three pitch areas at the image end portion is calculated, and a luminance difference of neighboring three pitch areas is calculated. An average luminance difference of ten spots having large luminance differences is calculated.

The display unit <b>1206</b> displays an SLO image obtaining rate, an OCT image, an eye movement measurement result, remaining measurement time, the image acquiring condition, and the like, and hence the user can confirm the operation.

As described above, the frame rate for obtaining the fundus images is determined from the luminance value of the template image. Thus, it is possible to increase speed of measurement of eye movement, and a higher resolution 3D retina tomographic image can be obtained by reflecting the eye movement on obtaining the OCT image.

Other Examples

In Examples 1 and 2, the template of blood vessels is extracted, but the same effect can be obtained by extracting a template of a macula lutea or a papilla. The fundus camera and the SLO apparatus are used for obtaining the fundus images, but another apparatus may be used for obtaining the fundus images. Even if the order of the steps in the flow are different from that described in the examples, or if another flow is used, the same effect can be obtained by determining the frame rate for obtaining the fundus images based on luminance of the template.

In the above-mentioned examples, after determining the frame rate, the initial template image is used for performing template matching between images obtained at different frame rates. However, the template may be extracted again after the frame rate is determined, and the template image after determining the frame rate may be used for performing the template matching between images obtained at the same frame rate.

In the method of calculating the template luminance, the graph illustrated in <figref idref="DRAWINGS">FIG. 8</figref> is used. However, the graph to be used may be changed based on other attribution information, so as to calculate the frame rate with higher accuracy. Thus, a further effect can be expected. Further, the dividing method as illustrated in <figref idref="DRAWINGS">FIGS. 10 and 14</figref> is used for selecting luminance of the template image, but another method of calculating luminance may be adopted. In addition, when performing reexamination of the same subject, a more accurate matching region can be set by using the image data (in particular, frame rate) of the previous measurement. The same effect can be obtained by using an SN ratio as the luminance information of the template.

The OCT apparatus is used in Example 2, but the same effect can be confirmed by using an ophthalmologic apparatus for a vision field test or the like. In addition, eye movement correction is performed in real time for the ophthalmologic apparatus, but the effect can be obtained also by performing the correction or post-measurement process after the measurement is finished.

In addition, the present invention is realized also by performing the following process. That is, software (program) for realizing the above-mentioned functions of the examples is supplied to the system or the apparatus via a network or various types of storage media, and the computer (CPU or MPU) of the system or the apparatus reads and executes the program.

REFERENCE SIGNS LIST

<b>1</b>: fundus camera

<b>50</b>: digital single-lens reflex camera

Other Embodiments

Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiment(s), and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiment(s). For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (e.g., computer-readable medium). In such a case, the system or apparatus, and the recording medium where the program is stored, are included as being within the scope of the present invention.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2010-137945, filed Jun. 17, 2011, which is hereby incorporated by reference herein in its entirety.